Typically, an output level of a vehicle sensor such as an acceleration sensor or a pressure sensor is very small and between a few millivolts to hundreds of millivolts, for example. In contrast, an analog-to-digital (A/D) converter built in a vehicle microcomputer typically has a dynamic range from 0 volts to 5 volts. Therefore, when the sensor output is A/D-converted by the A/D converter, the dynamic range cannot be used effectively. As a result, an A/D output of the A/D converter has poor precision.
As shown in FIG. 17, one approach to improve the precision of the A/D output is to provide an amplifier circuit with a suitable gain upstream of the A/D converter. As shown in FIGS. 18A, 18B, a small sensor output is amplified to a suitable level for A/D conversion by the amplifier circuit according to a kind of sensors, signal level of the sensor output, or the like. In such an approach, the dynamic range of the A/D converter can be used effectively so that the precision of the A/D output can be increased.
For example, in U.S. Pat. No. 5,892,472, multiple amplifier circuits (i.e., switched capacitor circuits) having adjustable gains are provided upstream of the A/D converter. However, the multiple amplifier circuits result in an increase in IC chip size. Further, since the multiple amplifier circuits are connected in series, errors such as an offset error, a gain error, and a linearity error are accumulated. As a result, the A/D output of the A/D converter may have poor precision. The errors may be reduced by adding correction circuits to the A/D converter. However, the addition of the correction circuits results in the increase in IC chip size.
Another approach to improve the precision of the A/D output is to increase resolution of the A/D converter. However, it is generally difficult to increase the resolution. In an A/D converter disclosed in JP-A-2005-260449, a conversion resolution is reduced according to an input signal so that quantization width is equalized. However, this approach does not use the dynamic range effectively and cannot increase the resolution.